The present invention relates to leak detection in an automotive evaporative emission system, and more particularly to a detection method that accurately detects a leak in a system including a flexible fuel tank.
In an automotive evaporative emission system, fuel vapor generated in the vehicle fuel tank is captured in a charcoal-filled canister and subsequently supplied to the engine air intake through a solenoid purge valve. Since the effectiveness of the system can be significantly impaired by faulty operation of a component or by a leak in one or more of the hoses or components, the engine controller is generally programmed to carry out a number of diagnostic algorithms for detecting such failures. If faulty operation is detected, the result is stored and a xe2x80x9ccheck enginexe2x80x9d lamp is activated to alert the driver so that corrective action can be taken.
Experience has shown that small leaks in an evaporative system can be particularly difficult to reliably detect. Theoretically, leaks as small as 0.5 mm (0.02 in.) can be detected by closing the vapor purge valve, evacuating the system to a predetermined vacuum level, and then monitoring the vacuum decay rate over a predetermined interval of time. See for example, the U.S. Pat. No. 6,308,119, issued on Oct. 23, 2001, assigned to the assignee of the present invention, and incorporated by reference herein. However, it has been found that the test data can be misinterpreted, particularly in systems where the fuel tank is sufficiently flexible that its contained volume changes during the diagnostic test. Specifically, the volume of the tank tends to increase as the system pressure decays toward atmospheric pressure due to a leak or fuel vapor generation, and this has the effect of reducing the observed decay rate. As a result, a small leak in the evaporative system may go undetected. Accordingly, what is needed is a method of reliably detecting evaporative emission system leaks in a system including a flexible fuel tank.
The present invention is directed to an improved method of testing for evaporative emission system leaks by monitoring vacuum decay in a closed system, wherein the effects of fuel tank expansion during the test interval are minimized. In a first embodiment, the pass/fail criterion is established in terms of the time required for the system pressure to decay by a calibrated amount corresponding to a predetermined leak size. A leak at least as large as the predetermined leak is detected if the measured time is shorter than a calibrated time. The effects of fuel tank expansion are minimized because the changes in fuel tank volume occur primarily due to the pressure differential across the tank, as opposed to the leak size, and the changes that occur during the test are essentially the same for any leak size under consideration. In a second embodiment, the pass/fail criterion is established in terms of the change in pressure that occurs in the calibrated time; a leak at least as large as the predetermined leak is detected if the measured change in pressure is larger than the calibrated pressure amount.